This invention relates to NTSC color television transmission (broadcast or cable), and/or recording for later transmission or display on a conventional NTSC monitor or television receiver, and more particularly to an improved sync generator for producing an NTSC composite color signal to be transmitted free from chroma crawl.
To make the frequency spectrum of color and luminance interleave, the NTSC frame and line frequencies were chosen in order to make the color subcarrier an odd multiple of half the frame and line frequencies. Each line contains 227.5 subcarrier cycles and each frame contains 119,437.5 subcarrier cycles. Each field ends with the subcarrier 90.degree. out of phase from the previous one. Consequently, color subcarrier cycles are present in the same phase at each consecutive scanning field one line higher. This causes the artifact known as chroma crawl. The standard NTSC subcarrier pattern is repeated every four fields, which introduces a 15 Hz component in the image display which can be noticeable and disturbing at edges of certain details in the picture.
Methods for producing composite color signals without chroma crawl in NTSC color television transmission are disclosed in U.S. Pats. Nos. 4,660,074 and 4,661,840. The methods suggested in those patents are based on slightly changing the length of horizontal scanning lines or fields at the source to end each scan line with one full color subcarrier cycle, thus shortening each raster (horizontal scan line) to 227 color subcarrier cycles, or to end each field with 59,718.5 cycles of the color subcarrier, instead of 59,718.75 cycles. As a consequence, there is a time differential between a full NTSC frame and a frame derived by the methods of these two patents.